1. Field of the Invention
The present invention relates to acoustic signal processing with adaptations for electromagnetic processing ranging from Extremely Low Frequency (ELF) to Ultra Violet (UV).
2. Description of the Related Art
The desire to achieve superior performance in acoustic data acquisition systems has been partially met by the construction of large arrays of acoustic sensors. Practical engineering constraints for portable sensor arrays have lead to the design and construction of flexible sensor array structures. Optimal processing of the data collected from flexible arrays of sensors requires detailed knowledge of the time variant positions of the sensors in the array. Some of the largest flexible arrays, and some of the largest rigid arrays, are now large enough that the non-isotropic characteristics of the environment are of concern when processing data from the sensor array. For example, with these large arrays, acoustic signals may propagate through a complex ocean environment with varying temperature, pressure and salinity characteristics, as well as differing physical contours and obstructions. Each of these parameters can affect the path, strength and condition of the transmitted and received signals. Accordingly, signal processing of such large array systems must account for the differing properties of the environment along axes in all directions.
For commercial systems some of this positioning and environmental data has been collected using acoustic projector to acoustic receiver propagation delay measurements using pulsed acoustic waveforms. These pulsed waveforms can preclude the simultaneous arrival of signals from multiple propagation paths, and can provide minimum power requirements for the acoustic projectors. Simple waveforms, such as pulsed CW and FM slides, minimize the required computational requirements for the delay measurements.
Military units also use large acoustic sensor array structures to maximize acoustic sensor performance. These systems also require sensor location and environmental data to support acoustic signal processing of the sensor array data. For obvious reasons, many of these military systems also desire that the sensor array not be detected by opposing forces. However, the traditional waveforms used for propagation measurements are not optimized to minimize the risk of detection by opposing forces. Most traditional solutions are challenged by the evolution of system capabilities. For example, the utility of very high frequency transmissions is reduced by scattering and absorption of the transmitted and received energy by the sensor array structure. The security provided by the environmental scattering and absorption between the projector and the sensor system employed by opposing forces is of decreasing utility as tactical engagement ranges shorten. The difficulty of opposing forces detecting very short transmissions (of a few cycles in duration) is now reduced by the falling cost of signal processing hardware. The use of short duration waveforms mimicking natural transient energy (such as biologic sources) has its own set of engineering and counter detection constraints.
Both the force deploying the sensor array, and the opposing force, desire to detect and classify (recognize as of interest) the energy transmitted to support the required propagation delay measurements. The signal processing advantage afforded the force deploying the array is knowledge of the transmitted waveform. This advantage should therefore be maximized. In addition, it is a basic tenet of search theory that detections are based on differences between a signal and background noise. The primary factors are Signal-to-Noise Ratio (SNR) and Recognition Differential. Traditional waveforms (CW pulses, FM slides, and the like) are frequently chosen because they are qualitatively distinct from background noise typically encountered in the marine environment, and thus contribute to increased recognition differential, so as to be easily identified by the intended processing system. However, a drawback is they are thus more detectable by an opposing force than a signal of similar SNR that blends with the marine background environment.